1. Field of the Invention
The present invention relates to filter assemblies and methods, and more particularly, to a dynamic swirl filter assembly for developing fluid shear boundary layers at the surfaces of the filters by rotating the process fluid and a method for providing enhanced filtration.
2. Discussion of the Prior Art
Filtration devices are utilized to separate one or more components of a fluid from other components. As used herein, the term "fluid" includes liquids and all mixtures of liquids and solids or liquids and gases that behave substantially as liquids. Common processes carried out in such devices include classic filtration, microfiltration, ultrafiltration, reverse osmosis, dialysis, electrodialysis, pervaporation, water splitting, sieving, affinity separation, affinity purification, affinity sorption, chromatography, gel filtration, and bacteriological filtration. As used herein, the term "filtration" includes all of those separation processes as well as any other processes utilizing a filter that separates one or more components of a fluid from the other components of the fluid.
A common problem in virtually all filtration systems is blinding or fouling of the filter, for example, a permeable membrane. Permeate passing through the filter from the fluid layer adjacent to the feed side or upstream portion of the filter leaves a layer of material adjacent to, or on the feed side of the filter having a different composition than that of the bulk process fluid. This layer by virtue of its composition may hinder transport of the components trying to pass through the filter to the permeate side of the filter or may include substances which can bind to the filter and clog its pores, thereby fouling the filter. Accordingly, mass transport through the filter per unit time, i.e., flux, may be reduced and the component separating capability of the filter may be adversely affected.
It is well known that a layer of fluid which is adjacent to the surface of a filter and which is in a state of rapid shear flow parallel to the surface of the filter tends to minimize fouling of the filter by the generation of lift on contaminant matter contained in the process fluid. Basically, the generation of lift on contaminant matter tends to reduce fouling of the filter by maintaining an obstruction free path through the filter. In other words, if little or no contaminant matter is on or near the surface of the filter, the process fluid flows directly into the filter wherein undesirable constituents are removed therefrom. Although the undesirable constituents may eventually foul the filter, the larger constituents in the process fluid, which would tend to foul the filter more quickly, remain suspended in the retentate. Accordingly, filter life is prolonged and permeate flow rate is improved.
Essentially, two categories of technology are currently utilized for developing a shear layer at the surface of a filter, cross flow filter systems and dynamic filter systems. In cross flow systems, high volumes of fluid are driven through passages bounded by the filter surface and possibly the inner surface of the filter housing, thereby creating the necessary shear. Simply stated, process fluid is pumped across the upstream surface of the filter at a velocity high enough to disrupt and back mix the boundary layer.
An inherent weakness common to cross flow filter systems is that a significant pressure drop occurs between the inlet and outlet of the filter system, and any increase in shear rate will be accompanied by an increase in this pressure drop. Specifically, the process fluid entering the filter system is under a great deal of pressure in order to develop high velocities; however, once the process fluid is dispersed across the filter elements comprising the filter system, the pressure sharply decreases. This decrease in pressure across the filter elements causes non-uniformity in transmembrane pressure, i.e., pressure differences between the upstream and downstream sides of the filter elements. Non-uniformity in transmembrane pressure tends to cause fouling of the filter elements in a non-uniform manner. Non-uniform fouling occurs because more contaminants may be deposited in a particular area which is subject to a higher process fluid pressure. Filter longevity and efficiency is reduced because certain areas of the filter elements may become fouled more rapidly than other areas, thereby leading to greater non-uniformity in transmembrane pressure and thus increased preferential fouling. Accordingly, the mechanism, i.e., high shear rate, for improving the performance of the filter results in a by-product, i.e., high pressure drop, which tends to reduce the performance of the filter. In addition, in cross flow filter systems, the high feed rates as compared to the filtration rates requires numerous feed recycles through the system, which are, in many processes, undesirable.
Dynamic filter systems overcome the excessive pressure differential problem associated with cross flow filter systems by supplying power to generate the shear flow through a moving surface rather than a pressure differential. Dynamic filter systems may be constructed in various configurations. Two widely used configurations are cylinder devices and disc devices. Within each of these two configurations, numerous variations in design exist. In cylinder devices, a cylindrical filter element is positioned in a concentric shell or filter housing. The shear layer is created in the gap between the filter element and the shell by spinning either the filter element or the shell about a common axis. The shear rate increases with both angular velocity and filter element radius. Conversely, the shear rate decreases as the gap between the filter element and the shell is increased. Accordingly, cylindrical filtering systems which are highly efficient due to high shear rates must either have small gaps which are difficult to manufacture, or large radii which limits the amount of filter surface area that may be packed within the filter vessel.
In disc devices a set of parallel filter discs interleaved with a set of impermeable discs are aligned along a common axis and positioned within the filter housing. In these devices shear is created by rotating the filter discs, or by rotating the impermeable discs. Disc devices overcome some of the disadvantages of cross flow and cylinder devices, but suffer from complexity of design. A major design difficulty is to provide sufficient mechanical support for the discs without either obstructing the fluid flow paths or making the distances between adjacent discs large. A large distance or gap between the discs is undesirable because it increases the overall size of the device and because it increases the amount of fluid retained on the inlet or upstream side of the filter surface, i.e., increases hold-up volume.